It has a huge effect on reverberation times, but much less on frequency response. Your perception of the change in frequency will be very great--particularly when you absorb at the first reflection points (notice the simulator doesn't have position as part of simulators--it's a very simple calculation being done there). So you may be missing the difference on the reverberation time graph. We recently did an article on acoustical measurements, you can find it on our resources page which may explain some of this in more detail for you.
resources
resources